The present invention relates to a damping force adjustable fluid pressure shock absorber in which damping force characteristics can be appropriately adjusted.
As a hydraulic shock absorber mounted on a suspension apparatus of a vehicle such as an automobile, there is known a damping force adjustable hydraulic shock absorber in which damping force characteristics can be appropriately adjusted in accordance with road surface conditions, vehicle running conditions, and the like, so as to improve ride comfort and steering stability.
Generally, a damping force adjustable hydraulic shock absorber comprises a cylinder in which a hydraulic fluid is sealingly contained, a piston rod, and a piston coupled to the piston rod. The piston is slidably fitted in the cylinder so as to divide the inside of the cylinder into two chambers. The piston includes a primary hydraulic fluid passage and a bypass passage through which the two chambers in the cylinder are in communication with each other. A damping force generating mechanism is provided at the primary hydraulic fluid passage. The damping force generating mechanism comprises an orifice, a disk valve and the like. A damping force adjusting valve is provided at the bypass passage. The damping force adjusting valve adjusts the passage area of the bypass passage.
To reduce a damping force, the bypass passage is opened by use of the damping force adjusting valve so as to reduce resistance to a hydraulic fluid flow between the two chambers in the cylinder. To increase a damping force, the bypass passage is closed by use of the damping force adjusting valve so as to increase resistance to a hydraulic fluid flow between the two chambers. In this way, damping force characteristics can be appropriately adjusted by opening or closing the damping force adjusting valve.
However, in a hydraulic shock absorber in which damping force adjustment relies, as described above, on changing only a passage area of a bypass passage, it is possible to change damping force characteristics to a large extent in a low piston speed range. This is because in this range a damping force depends on an orifice dimension of a hydraulic fluid passage. However, it is not possible to change damping force characteristics to a large extent in middle and high piston speed ranges. This is because in these ranges a damping force depends on an opening degree of a damping force generating mechanism (for example, disk valve) provided at a primary hydraulic fluid passage.
With the aim of solving the drawback, for example, Japanese Patent Application Public Disclosure 2003-278819 discloses a damping force adjustable hydraulic shock absorber provided with a pilot type damping valve. In the pilot type damping valve, a backpressure chamber (pilot chamber) is formed at the back of a main valve (disk valve) of a primary hydraulic fluid passage, and the backpressure chamber is in communication through a fixed orifice with a cylinder chamber of an upstream side of the disk valve, and is also in communication through a damping force adjusting valve (pilot control valve) with a cylinder chamber of a downstream side of the disk valve.
According to this damping force adjustable hydraulic shock absorber, the area of the communication passage between the two chambers in the cylinder can be directly adjusted by opening or closing the damping force adjusting valve, and at the same time, the valve-opening pressure of the main valve can be changed by changing the pressure of the backpressure chamber by utilizing pressure loss caused by the damping force adjusting valve. In this way, it is possible to adjust not only orifice characteristics (in which a damping force is approximately proportional to the square of piston speed) but also valve characteristics (in which a damping force is approximately proportional to piston speed), thereby to widen an adjustable range of damping force characteristics.
However, the above-mentioned conventional damping force adjustable hydraulic shock absorber provided with the pilot type damping valve remains subject to the following drawback. Namely, when hydraulic fluid passes through the damping force adjusting valve and enters the cylinder chamber of the downstream side, because the cylinder chamber is being depressurized, the inside of it is liable to a sudden drastic pressure change and therefore bubbles are easily created in the hydraulic fluid, with the possible consequence that an unstable damping force might be generated.